Inclusions are oxides or sulfides in steel which have a detrimental effect on mechanical properties of the steel such as ductility, fracture toughness, fatigue strength, and stress corrosion resistance. It is known that the detrimental effect of inclusions can be significantly reduced if the shape of the inclusions can be controlled such that the inclusions are of generally spherical shape rather than of long and thin shape. Such shape control is achieved by adding substances to the steel which combine with the normal oxide and/or sulfide forming elements to form complex inclusions which are essentially spherical in shape and which maintain their shape during hot working operations.
One additive which may be added for inclusion shape control is calcium. However, calcium has disadvantages which have heretofore detracted from its utility as an inclusion shape control additive.
Calcium has a relatively high vapor pressure at steelmaking temperatures and a relatively low density compared to molten steel. Furthermore it has relatively limited solubility in steel. Therefore it is very difficult to effectively provide the requisite amount of calcium to the steel to successfully modify oxide and sulfide inclusions to control their shape. Calcium tends to volatize rather than be dissolved in a steel bath because of its high vapor pressure. Calcium also tends to float out of the steel melt and into the slag before it can dissolve due to its limited solubility and low density. Consequently, specialized and expensive techniques are employed in order to successfully employ calcium as an inclusion shape control additive. One technique is the injection of powdered calcium containing compounds deep below the surface of the melt in the ladle. This technique has disadvantages because the required injection equipment is expensive and costly to maintain, the injection process results in a temperature loss to the melt and the injection process inevitably introduces unwanted nitrogen, oxygen and hydrogen to the steel from the air over the splashing melt. Another technique involves the introduction of calcium to the melt as cored wire, i.e., calcium metal encased in a steel sheath. The disadvantages of this technique are the high cost of cored wire and difficulty in effectively treating large batches of steel due to problems in penetrating the slag layer which is usually present as well as limitations on the rate at which wire can be added.
Calcium, despite these disadvantages, is generally the preferred additive for inclusion shape control. This is because calcium modifies oxide and sulfide inclusions to give excellently shaped inclusions which are very uniformly distributed throughout the steel. Moreover, the use of calcium does not adversely affect total inclusion content and reduces the tendency of some steels to clog nozzles during casting operations. Thus one can achieve a steel having good mechanical properties and superior castability because the inclusions have been modified by calcium addition, albeit at a high cost.
It is therefore desirable to provide a method which will allow calcium to be used as an inclusion shape control additive without need to resort to expensive and complicated methods to successfully add sufficient calcium to the melt.
It is an object of this invention to provide an improved method to control the shape of inclusions in steel.
It is another object of this invention to provide an improved process for the production of steel wherein calcium can be employed to control the shape of inclusions.
It is a further object of this invention to provide a process for the production of steel wherein calcium can be employed to control the shape of inclusions and can be successfully added to the steel melt without need for complicated or expensive addition techniques.